The present invention relates to a system, method and apparatus for handling tip/ring faults in a ringing SLIC and in particular for providing desired safety features following a fault, while achieving increased loop length and/or supporting a high ringer equivalent number (REN) load.
In older telephone systems, users"" telephone units were coupled to a telephone exchange by transformers or similar devices. In current telephone systems, it has become common to provide coupling to an exchange by a system which includes a xe2x80x9cline cardxe2x80x9d, commonly having a subscriber line interface circuit (SLIC). A number of different types of SLIC devices are in use. In some configurations, the line card includes a ring-voltage source (for ringing subscriber units which are on the xe2x80x9cloopxe2x80x9d) which is separate from the SLIC. However, in a xe2x80x9cringing SLICxe2x80x9d, the ringer voltage source is integrated within the same silicon/IC package as the SLIC.
In general, it is desired to provide telephone systems and equipment which can substantially assure the safety of users, technicians and the like. Many governmental or private organizations have established requirements or guidelines which help assure various safety standards. One common safety standard relates to the maximum voltage (or, in some cases, current) present on a tip or ring line, e.g., of a xe2x80x9cplain old telephone servicexe2x80x9d (POTS) line, after a single fault has occurred on either the tip or ring line. Among the standards directed to this safety feature are underwriters laboratories (UL) 1950 and Canadian Standards Association (CSA) C22.2 No. 950 (incorporated herein by reference). While standards relating to maximum voltage after a single fault may vary slightly from country to country and/or may be revised over time, in general, it is the consensus that safety, in this regard, (and compliance with most standards) is assured if, following a single fault, current on the tip or ring line does not exceed about 20 milliamps. A general purpose of the standard is to assure safety of humans who may come in contact with the tip or ring line in those conditions. In general, it is reasonable to model the human body by a resistance of about 5 kilohms, leading to a general requirement that the voltage on the tip or ring line, after a single fault, should not exceed about 90 to 100 volts.
As noted above, in a ringing SLIC device, the ringer voltage source and the SLIC are integrated within the same Silicon/IC package. Although there may be some ringer SLIC devices which do not consistently comply with the maximum voltage/current standards, the previous approach to providing a line card which does comply was typically to provide or design a ringing SLIC which is incapable of outputting ringer voltage in excess of about 90 to 100 volts (at any time). Limiting the maximum ringer voltage in this manner places effective constraints on the ringer equivalent number (REN) load which can be supported and/or the loop xe2x80x9creachxe2x80x9d that can be achieved, e.g. on a POTS line. For example, in some previous approaches, a ringing SLIC could not support more than about a three REN load. As described in Federal Communications Commission (FCC) regulation Part 6 8.313 (incorporated herein by reference), a REN can be affected by, among other things, the ringing frequency. Accordingly, it would be useful to provide a system, method and apparatus for making or using a ringing SLIC which can provide the required or desired safety standards while supporting an increased REN load and/or a longer loop reach.
In addition to safety standards, including those noted above, governments and private organizations have also developed system integrity and/or performance standards. For example, various standards may require that equipment coupled to tip and ring lines must not perturb certain electrical characteristics of the tip and ring lines outside certain tolerance parameters. For example, in some standards there are limits on the amount of impedance change which can be made by adding or coupling equipment to tip and ring lines.
Accordingly, it would be useful to provide a method, system and apparatus for assuring compliance with safety requirements or otherwise assuring user safety, without substantially perturbing electrical characteristics on the tip and ring lines.
The present invention includes a recognition of the existence, nature and/or source of problems in previous approaches, including as described herein. In one aspect, a feedback system is provided to assure compliance with safety standards, or otherwise assuring safety, particularly after occurrence of at least a single fault on the tip or ring line, but which permits relatively high ringer voltage and/or current, in normal use, e.g. as long as there is no fault detected. In this way, relatively high voltages, such as greater than about 80 volts, more preferably greater than 90 volts, and even more preferably greater than 100 volts, can be provided during normal operation but, in the event of detection of a fault on the tip or ring line, the amount of voltage can be reduced so as to assure compliance with safety standards or otherwise assure safety of users. Preferably, monitoring or xe2x80x9csnoopingxe2x80x9d of the tip and ring lines is done in a manner such that there is little real effect on the electrical characteristics on the tip and the ring line, such as by measuring voltage and/or current on the tip and ring lines.
The system can be configured in a number of ways to provide an appropriate response to an excess of voltage or current on the tip or ring lines. In one embodiment, a current and/or voltage sensor is provided external to the ringing SLIC and, in response to the detection of an excess of voltage or current on the tip or ring lines, a power supply is controlled to reduce the amount of power provided to one or more pins of the SLIC so as to effectively reduce the amount of voltage output as a ringer signal. In one embodiment, detection of excessive voltage or current on the tip or ring lines, and/or control or reduction of the ringer voltage, is performed internally in the ringer SLIC. Although it is possible to provide logic, used for making the appropriate response to tip/ring current or voltage magnitudes, internally to the line card and/or SLIC, it is also possible to use an external, e.g., general purpose, CPU (xe2x80x9csystemxe2x80x9d CPU) for receiving indications of tip or ring current or voltage and outputting control signals so as to obtain the desired result.
In one aspect, a line card or other telephone or communications interface provided with a ringing SLIC outputs relatively high voltage, such as in excess of about 90 volts, and/or relatively high current such as in excess of about 20 milliamps during normal use, but decreases the voltage or current of a ringing signal or other signal upon occurrence of a tip and/or ring line fault or other abnormal condition. The abnormal condition is detected by measuring voltage and/or current on the tip line and/or ring line. In response to an excess voltage or current, the ringing current or other output current is reduced to achieve desired safety and/or compliance with standards.